This invention relates to semiconductor wafer processing, and more particularly to a semiconductor wafer susceptor which can be used in batch processing of semiconductor substrates.
For common semiconductor films such as silicon nitride, polysilicon, and thermal oxides, substrate processing usually proceeds by elevating the substrate to some process temperature, conducting the process, and finally cooling the substrate. Generally, most processes are conducted in a 200 mm batch furnace where substrates (hereafter referred to as wafers) are placed in a vertically stacked arrangement. Because of process and throughput requirements, the wafer stack often undergoes rapid heating and cooling at the beginning and end of the process. However, some thermal ramping limits exist at higher processing temperatures. It is now known that for 300 mm wafer, serious limitations exist on wafer heating/cooling rates and maximum process temperatures, well below the operational limits of the processing equipment.
The gravitational force and elevated process temperature (typically above 850° C.) cause considerable stress on the silica on a wafer, leading to situations where slip and plastic deformation may occur. Fast thermal ramping can further degrade the situation because within-wafer (WinW) thermal gradients from uneven heating of wafers in a vertical stacked arrangement may cause slip to occur even before the process temperature is reached. Of course, fast thermal ramping is employed to increase productivity by decreasing the overall cycle time or reduce thermal budget by decreasing the ramping cycles. Therefore, a serious situation arises for high temperature processing of 300 mm substrate, especially in batch processing environments. Additionally, even if slip does not occur, the induced thermal gradient on the wafer may be of sufficient magnitude as to cause significant differences in the thermal histories of the die spread across the wafer. This will result in an unexpected die performance variation between the wafer center and edge locations.
Two approaches can be taken to solve this slip problem. One approach is to improve the wafer's chemical and mechanical characteristics, such as decreasing the oxygen precipitate concentration within the silicon wafer. This approach is an area of responsibility for the wafer manufacturers. The other approach is to improve the substrate support design.
The current industry standard for vertical batch wafer processing is the ladder boat and its variations (FIG. 1). This is the simplest design for vertical batch processing. However, it does not provide the most optimum mechanical support possible with respect to gravitational forces. Also, the standard ladder boat provides little reduction in thermal gradients. The ladder boat's greatest advantages are its low cost and compatibility with standard automation.
Two previously developed innovations have addressed the WinW wafer thermal issue for batch processing. The first wafer support method, shown in FIG. 2, was developed and patented by Tokyo Electron Ltd. (TEL). This “ring” support method uses a ring of material (typically quartz) designed to come into physical contact with the edge of the wafer. The addition of mass near or at the wafer's edge reduces the WinW thermal gradient because of the increase in heat capacity and change in radiation view factors. The method also provides a larger area of mechanical support than a ladder boat. The method gives good performance on 200 mm wafers, as thermal WinW gradients are controlled to under 10° C. for fast thermal ramps (above 75° C./min). However, this support method is complex and such designs are more expensive to manufacture and purchase. Additionally, this method requires more complex automation to load and unload wafers from the support appliance, leading to added cost for the associated support automation.
Another approach found in the prior art (previously patented by SVG, Thermco Systems) is the “band” method as shown in FIG. 3. Here, a thin band of material, typically quartz, is placed around the edge of the wafer, but not in intimate contact. The quartz material is either opaque or mechanically modified to be translucent. This method, like the ring support, reduces or screens incident radiation onto the wafer's edge, while permitting radiation through the unblocked areas and onto the wafer's center. Although not as effective as the ring support method shown in FIG. 2, the “band” method does reduce WinW thermal gradients and can be manufactured at a lower cost.
Other approaches to wafer support methodologies have been previously explored by others and are well known within the industry. In FIG. 4A, the best theoretical point contact support at a single radius value is shown. This method places point supports at 70% of the radial distance from the center to the wafer's edge, to balance the weight of the wafer on either side of the support and reduce gravitational stress effects. This approach when implemented in a ladder boat configuration will provide better support, but the cost will be greater due to the additional manufacturing complexity of very long support tabs. Also, this method does not address the WinW thermal gradient problem. A corresponding analogy exists for the ring support (point contact) where the location for a single ring would be also at 70% of the radial distance from the center to the wafer's edge. In this case, the ring support's axial symmetry greatly improves the control of the gravitational stress magnitude and symmetry compared to the ladder boat method. FIG. 4C shows the absolute best theoretical support design possible, as all points on the wafer are mechanically supported. Clean, simple, and efficient mechanical wafer loading and unloading for this design becomes a serious problem, if not impossible, with current automation technology.
The vast majority of single wafer processing equipment currently use supports shown in FIGS. 4B and 4C. Here either a ring of material or a flat plate or susceptor composed of quartz, SiC or similar material supports the wafer. These designs are preferred for reasons of simplicity or reduction of thermal mass to permit rapid wafer heating and cooling (up to 100° C./sec). The supports in FIGS. 4B and 4C are not necessarily employed for thermal WinW control in single wafer processing equipment because they rely on heating element design to accomplish WinW thermal uniformity. In some cases, there may be some benefit based on material selection with reducing thermal non-uniformity. As an added benefit, gravitational forces are reduced and in the case of FIG. 4C, are completely eliminated if the right support material is used. However, these designs do add complexity to the method of wafer handling and are best suited for single wafer environments where the automation comprises a larger percentage of the overall equipment set and cost.
WinW Thermal Gradients
The primary issue with batch processing and rapid heating of large substrates is the resultant thermal gradients, as demonstrated in FIG. 5. During the heating phase of the process cycle (see FIG. 5A), the edges of the wafer receive the majority of the incident radiation and as a result heat up at a faster rate. Heating of the interior regions of the wafer is chiefly accomplished by thermal conduction through the substrate itself. As a result, a “bowl”-shaped thermal profile forms across the wafer. This thermal gradient can add to the gravitational stress and—if large enough—cause warping, bowing, plastic deformation, and slip to occur. A solution to this problem would be to increase the pitch of the wafer stack, thereby increasing the radiation view factor for the wafer center.
As in the case for heating, rapid cooling of the wafer (see FIG. 5B) can also have negative effects. Efficient radiative cooling of the wafer's edge occurs because of a large exposed area (large angular exposure to the heater walls) at the wafer's edge. The interior regions of the wafer have smaller exposed angular area to the outside and thus cool inefficiently through radiation. The central region of the wafer mainly cools through thermal conduction from the wafer center to the edge where then energy is more effectively radiated away. As a result, a “dome”-like thermal gradient is formed across the wafer. This thermal gradient can add to the gravitational stress and—if large enough—cause warping, bowing, plastic deformation, and slip to occur. Like heating, a solution would be to increase the radiation view factor for the wafer center.
Given a particular support design, the magnitude of this WinW thermal gradient coupled with the process temperature determines whether slip conditions exist. FIG. 6 shows the difference in slip curves between a ladder-type and a ring boat. For a given WinW thermal gradient and process temperature, the wafer will tend to exhibit slip if the process condition lies on the right-hand side of the slip curve. FIG. 6 shows that the maximum allowable delta T decreases rapidly with increasing wafer edge temperature.
As seen in FIG. 7, the ladder boat (3point support) would not be sufficient for processes requiring temperatures above 850° C., as slip and possible plastic deformation would occur. Increasing the number of point supports for a ladder boat or decreasing the oxygen precipitate concentration would help. Increasing the number of point supports and relocating them to the optimum locations would shift the slip curve to the right and permit a larger allowable WinW thermal gradient the processing temperature. The disk and ring supports lie near the limit for such improvements. The ring boat (ring support) would be adequate for the high temperature processes, but the complexity of wafer automation would be a disadvantage.